This invention relates to mirrors and more particularly to a mirror having means for compensating thermal distortions therein resulting from heat generated by the absorption of radiation at the reflective surface of the mirror.
Mirrors adapted for use with high power laser beams are subject to severe thermal gradients resulting from the absorption of a portion of the high power beam at the reflective surface of the mirror. The thermal gradients result in a distortion of the mirror with a corresponding distortion in the wavefront of the radiation reflected from the reflective surface.
One method well known in the art for minimizing thermal distortion within mirrors is to flow a coolant therethrough in heat exchanger relationship with the mirror. Typically, either the faceplate of the mirror or the mirror substrate is cooled to minimize the temperature variations within the mirror. For example, Staley in U.S. Pat. No. 3,645,608 filed May 5, 1970 discloses a mirror having a reflective surface formed on a flat side of a channel plate which is attached to a substrate and a coolant manifold system disposed within the channel plate for passing coolant therethrough in heat exchanger relationship with the channel plate and substrate. McLafferty et al in U.S. Pat. No. 3,637,296 filed June 4, 1970 also discloses a mirror, including a manifold system disposed proximate a reflective surface, for passing coolant therethrough. These and similar prior art devices typically utilize the coolant for minimizing the temperature variations in the mirror during operation thereby minimizing thermal deformations. However, these prior art cooled mirrors do not provide for compensating the thermal distortions in the mirror resulting from the absorption of heat therein.
A mirror adapted for compensating thermal distortions is disclosed by Hofnagel in U.S. Pat. No. 3,609,589 filed Sept. 3, 1968. The mirror is formed by bonding a plurality of plates of differing materials to form the mirror substrate. A top plate having one side optically polished provides the reflective surface of the mirror. Each of the plates forming the substrate is selected to have a thickness and a coefficient of thermal expansion such that the tendency of the mirror to warp in one direction due to thermal gradients is opposed by and preferentially balanced in an opposite direction by the differential expansion of the additional plates such that the selected configuration of the mirror is retained over a range of temperature gradients. Typically the back plate of the mirror is formed of a material having a substantially higher coefficient of an expansion than the material of the top plate. The greater thermal expansion of the back plate tends to cause the mirror to deform with a curvature opposite to the curvature resulting from the thermal yield distortion of the top plate. This mirror is limited to prescribed restorative deformation and does not have the capability of responding to variations in the intensity distribution or energy level of an incident beam of radiation to compensate the resulting variation in the distortions of the top plate.